The present invention relates to a device for the production of fixed nitrogen and, more particularly, to a device for producing nitrogen oxides by an electric arc discharge process.
Nitrogen is an essential material in the production of fertilizers. While it is the major component of the atmosphere (79 percent in dry air), nitrogen can be incorporated into most living systems only in the "fixed" form and nitrogen is less abundant in its fixed form. For agricultural purposes, it is desirable to supplement the natural sources of fixed nitrogen with chemical fertilizers. Typically, chemical fertilizers contain nitrogen which is fixed by industrial methods in which nitrogen is combined with hydrogen to form ammonia.
The principal industrial method for producing ammonia is the Haber process. In the Haber process, one molecule of nitrogen and three molecules of hydrogen combine at elevated temperature and pressure in the presence of a catalyst to form two molecules of ammonia. The hydrogen utilized in the Haber process is obtained primarily from natural gas and liquid hydrocarbons. As long as there is a ready and inexpensive supply of hydrogen, the Haber process is unequaled in cost and efficiency for producing fixed nitrogen fertilizers. However, because of waning supplies of natural gas and oil, the source of supply of hydrogen has decreased, and there has been a concomittant rise in its price. The demand for fixed nitrogen continues to grow, however, due to world population increases and the introduction of nitrogeneous fertilizers in the underdeveloped regions of the world. Thus it would appear likely that the cost of fixed nitrogen will continue to increase.
Accordingly, an investigation has begun into nitrogen fertilizer production methods other than the Haber process. See, for example, Safrany, "Nitrogen Fixation", Scientific America, Vol. 231, No. 4, pp. 64-80 (1974), wherein the following possible alternatives are discussed: biological fixation, metallo-organic, thermal activation, and low temperature ionization.
Many of these alternatives strive to produce various nitrogen oxides, which with water addition will form nitric acid (HNO.sub.3). That is, depending on conditions, the reaction of nitrogen gas and oxygen gas will form one or more of the following nitrogen oxides: NO, N.sub.2 O.sub.3, NO.sub.2, or N.sub.2 O.sub.4. Safrany states that it is easiest to discuss the reaction as producing nitric oxide EQU (N.sub.2 +O.sub.2 =2NO)
It should be realized, however, that nitric oxide (NO) readily combines with oxygen at room temperature in an exothermic reaction to form nitrogen dioxide (NO.sub.2). Thus, the reaction N.sub.2 +20.sub.2 =2NO.sub.2 can be said to be favored since nitrogen dioxide has the lowest heat of formation.
In any event, for production of the nitrogen oxides, Safrany finds low-temperature ionization to be the most attractive alternative. He states:
"Low-temperature ionization has the significant advantage that in princple all the molecules of the gas can be ionized or excited. The activation can be accomplished by subjecting the air to an electrical potential of a few thousand volts, so that a low-temperature discharge is initiated, or by exposing air to an intense flux of ionizing radiation inside a nuclear reactor. In either case the gas molecules are bombarded by fast-moving ions and the collisions are inelastic. The resulting cascade of reactions can produce a substantial yield of nitrogen oxides."
The basics of using an electrical arc discharge for production of nitrogen oxides are well known. See, for example, Ephram, Inorganic Chemistry, Fifth Edition Revised, 1949, pp. 680-704. However, the art has also long recognized that difficulties exist with the arc discharge process. Thus in Ephram at page 683 it is stated:
The percentage of nitric oxides in the equilibrium N.sub.2 +O.sub.2 =2NO is:
______________________________________ Temperature (.degree.F.) Percent ______________________________________ 1500.degree. 0.1 2000.degree. 0.61 2500.degree. 1.79 2900.degree. 3.20 3200.degree. 4.43 4200.degree. 10.00 ______________________________________
In order to obtain a fair yield an exceptionally high temperature must be employed; 4200.degree. F. corresponds approximately to that attained in the electric arc, and a favorable yield can then be obtained. At this temperature, however, not only the establishment of the equilibrium, but also the back decomposition, is very rapid, and it is necessary to bring the nitric oxide formed to a region of lower temperature as quickly as possible to avoid a great part of it being lost. This is carried out by having the arc suitably constructed, so that either it is spread out by an electro-magnet into a thin disc of flame, through which the N-O mixture (air) is blown, or the arc is kept in motion in the form of a sinuous, narrow, spiral band, or is forced into a water-cooled iron tube. In this way, on a laboratory scale, up to 8 percent of the mixture has been converted to nitric oxide, and in technical operations, up to 2.5 percent. It is not only the thermal effect of the arc which is responsible for the formation of nitric oxide; under the influence of strong electric fields (silent discharges), oxygen and nitrogen are decomposed into atoms which can then combine to form nitric oxide. This process must also play a part in the arc process."
A number of arc reaction chamber designs have been proposed for production of nitrogen oxides. As shown in U.S. Pat. No. 4,010,897, issued Mar. 8, 1977, to Treharne et al, assigned to the assignee of the present invention, an arc discharge device may typically comprise a casing defining a cylindrical chamber in which is positioned a discharge electrode. The discharge electrode is electrically insulated from the chamber and when sufficient electrical potential is applied between the discharge electrode and the casing, electrical arcing in the chamber results. Air is supplied to the chamber through an inlet adjacent the discharge electrode mounting and passes along the discharge electrode to an outlet at the opposite end of the chamber. Air containing the nitrogen oxides produced by the discharge process, as discussed above, is removed from the chamber through the outlet and may thereafter be inserted into solution in water or an alkaline solution. An alternative discharge chamber design is disclosed in which the chamber is generally conical in shape such that the discharge electrode and the interior chamber surface define a spark gap which increases in dimension toward the chamber outlet opening. Arcs between the electrode and the casing move toward the outlet opening, resulting in a pumping action which tends to draw air into the chamber and move the gases in the chamber toward the outlet opening.
U.S. Pat. No. 1,453,435, issued May 1, 1923, to Buettner, and U.S. Pat. No. 3,666,408, issued May 30, 1972, to Grosse et al, disclose arc discharge devices for production of nitrogen oxides in which the discharge electrode extends downward in the discharge chamber. In the Buettner chamber arrangement, the bottom of the chamber is filled with water, with the arcing occurring between the discharge electrode and the water. The nitrogen oxides which are formed as a result of arcing are absorbed into the water.
One problem which is encountered with arc discharge reactors is initiation of arcing at start up of the reactor devices. If the arc air gap is substantial, it should be appreciated that a rather large electrical potential must be applied to the discharge electrode in order for the break down potential of the air in the arc path to be exceeded. Once an arc is established, and gases in the arc path are ionized, however, a substantially smaller discharge potential is required to maintain the arcing process. In order to reduce the power requirements for the discharge chamber, it is desirable to operate the chamber at a relatively low discharge potential. One arc discharge chamber arrangement, illustrated in U.S. Pat. No. 1,130,941, issued Mar. 9, 1915, to Summers, utilizes a separate power supply for providing a high voltage potential to the discharge electrodes at initiation of arcing, with a lower potential power source being provided for maintenance of the arc. It will be appreciated that such a dual power source unduly complicates the arc discharge device.
Another problem associated with arc reactor devices, of the type to which the present invention is directed, is production of a relatively nonturbulent air flow of substantial volume through the arc chamber. If the air flow is sufficiently turbulent, the arc will be blown out. Thus, the flow rate through discharge chambers utilized in prior art has been limited, since a high flow rate produced turbulent flow which tended to extinguish electrical arcing.
Accordingly, it is seen that there is a need for an improved arc discharge reactor for production of nitrogen oxides in which the device is simple in construction, and start up is facilitated while maintaining a high flow rate of air through the discharge chamber.